Roots pump and exhaust method

ABSTRACT

According to one embodiment, a roots pump includes a first roots pump, a second roots pump, and a connection passage. The first roots pump includes a first casing which has a first pump chamber. The second roots pump includes a second casing which has a second pump chamber. The connection passage connects an exhaust hole which is provided in the first pump chamber to an intake hole which is provided in the second pump chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2012-136799, filed on Jun. 18, 2012; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a roots pump and an exhaust method.

BACKGROUND

In a manufacturing process of a semiconductor device, a multi-stage roots pump is used as one means for causing a process room to become a vacuum state. The multi-stage roots pump has a configuration in which a gas is transferred by rotors respectively provided in a pair of rotary shafts, is compressed by pump chambers of which the volumes sequentially decrease, and is exhausted to the outside.

Since the gas is compressed in the respective pump chambers of the multi-stage roots pump, heat is generated. As a result, the temperature increases as it goes toward the exhaust-side pump chamber. The heat is transmitted to the rotor and the rotary shaft, so that the temperatures of the rotor and the rotary shaft increase and the temperature of the casing also increases. Since the outer surface of the casing contacts the external air, an increase in temperature thereof may be suppressed. One rotor and the rotary shaft are provided inside the pump chamber and the pressure of the pump chamber is lower than the atmospheric pressure. Accordingly, the heat radiation amount is smaller than that of the casing, and the temperature increasing degree is larger than that of the casing. Specifically, when the temperature increasing degree caused by the heat increases, a temperature difference between the casing and the portion including the rotor and the rotary shaft increases. As a result, the thermal expansion difference between the rotary shaft and the casing increases, and the rotary shaft becomes distorted by the heat, so that the rotor contacts the casing. Further, in a case where the multi-stage roots pump is used for the exhaust of the vacuum apparatus such as an etching apparatus using a corrosive gas, the exhaust side (high pressure side) rotor is corroded by the high-temperature gas, so that the surface state is degraded.

If the pump is stopped in a state where the temperature of the rotary shaft is high and the surface of the rotor is degraded, there is a problem in which the pump may not be operated due to the contact between the rotor and the casing even when an operator tries to activate the pump again.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top cross-sectional view schematically illustrating an example of a configuration of a roots pump according to an embodiment;

FIG. 2 is a cross-sectional view taken along the line A-A of FIG. 1; and

FIGS. 3A and 3B are diagrams schematically illustrating a state of a temperature of a pump chamber during the operation of the roots pump.

DETAILED DESCRIPTION

In general, according to one embodiment, there is provided a roots pump including a first roots pump, a second roots pump, and a connection passage. The first roots pump includes a first casing which has therein a first pump chamber, a first gas transfer member which is disposed inside the first pump chamber and rotates so as to transfer a gas in a predetermined direction, and a first support member which supports the first gas transfer member. The second roots pump includes a second casing which has therein a second pump chamber, a second gas transfer member which is disposed inside the second pump chamber and rotates so as to transfer a gas in a predetermined direction, and a second support member which supports the second gas transfer member. The connection passage connects an exhaust hole which is provided in the first pump chamber to an intake hole which is provided in the second pump chamber.

Hereinafter, referring to the accompanying drawings, the roots pump and the exhaust method according to the embodiment will be described in detail. Furthermore, the invention is not limited to the embodiment.

FIG. 1 is a top cross-sectional view schematically illustrating an example of a configuration of the roots pump according to the embodiment, and FIG. 2 is a cross-sectional view taken along the line A-A of FIG. 1. As illustrated in FIG. 1, a roots pump 10 has a configuration in which a low pressure side roots pump 10L as a first roots pump provided near a vacuum apparatus and a high pressure side roots pump 10H provided near an exhaust are connected to each other by a connection passage 41 which connects a pump chamber 16 c at the most downstream side in the gas transfer direction of the low pressure side roots pump 10L to a pump chamber 16 a at the most upstream side in the gas transfer direction of a high pressure side roots pump 10H.

In the description below, since the low pressure side roots pump 10L and the high pressure side roots pump 10H have the same structure, the description will be made by giving the same reference numerals to the same components.

Each of the low pressure side roots pump 10L and the high pressure side roots pump 10H includes an upstream side wall 11 and a downstream side wall 13 which are away from each other. The upstream side wall 11 is provided with a motor chamber 11 a which accommodates a motor 23, and the upstream end of the motor chamber 11 a in the gas transfer direction is covered by an upstream cover 12. A bearing 25 which rotatably supports a main shaft 21 a is supported inside the motor chamber 11 a.

At the downstream side of the downstream side wall 13 in the gas transfer direction, there are provided a gear 24 which is attached to the main shaft 21 a, a gear (not illustrated) which is attached to a driven shaft 21 b (FIG. 2), and a gear chamber 13 a which accommodates a lubricant and the like, and the downstream end of the gear chamber 13 a in the gas transfer direction is covered by a downstream cover 14. A bearing 26 which rotatably supports the main shaft 21 a is supported inside the downstream side wall 13.

A partition wall constituting member 15 is disposed between the upstream side wall 11 and the downstream side wall 13. The partition wall constituting member 15 includes a plurality of partition walls 15 a to 15 d and an outer wall 15 e which supports the partition walls 15 a to 15 d. A space which is surrounded by each of the partition walls 15 a to 15 d and the outer wall 15 e becomes each of the first to third pump chambers 16 a to 16 c. Specifically, the space which is surrounded by the partition walls 15 a and 15 b and the outer wall 15 e becomes the first pump chamber 16 a, the space which is surrounded by the partition walls 15 b and 15 c and the outer wall 15 e becomes the second pump chamber 16 b, and the space which is surrounded by the partition walls 15 c and 15 d and the outer wall 15 e becomes the third pump chamber 16 c. Furthermore, the upstream side wall 11, the upstream cover 12, the downstream side wall 13, the downstream cover 14, and the partition wall constituting member 15 constitute the casing.

Inside the low pressure side roots pump 10L and the high pressure side roots pump 10H, an intake hole 31 is provided in the partition wall constituting member 15 constituting the first pump chamber 16 a and an exhaust hole 33 is provided in the partition wall constituting member 15 constituting the third pump chamber 16 c. Further, the partition wall 15 b between the first pump chamber 16 a and the second pump chamber 16 b and the partition wall 15 c between the second pump chamber 16 b and the third pump chamber 16 c are respectively provided with communication holes 32 a and 32 b. In this way, the intake hole 31 is connected to the exhaust hole 33 inside the same roots pump by the first pump chamber 16 a, the communication hole 32 a, the second pump chamber 16 b, the communication hole 32 b, and the third pump chamber 16 c, and the direction from the intake hole 31 to the exhaust hole 33 becomes the gas transfer direction.

The volumes of the respective pump chambers 16 a to 16 c are formed so as to become smaller from the upstream side toward the downstream side in the gas transfer direction. Further, the volume of the first pump chamber 16 a at the upstream side of the high pressure side roots pump 10H is formed so as to become smaller than the volume of the third pump chamber 16 c at the downstream side of the low pressure side roots pump 10L.

As illustrated in FIGS. 1 and 2, a pair of shafts, that is, the main shaft 21 a and the driven shaft 21 b which penetrate in parallel the upstream side wall 11, the downstream side wall 13, and the respective partition walls 15 a to 15 d of the partition wall constituting member 15 are rotatably supported, and the main shaft 21 a is rotationally driven by the motor 23. The main shaft 21 a and the driven shaft 21 b are attached with the gear 24 meshing with another gear inside the gear chamber 13 a (in FIG. 1, the gear which is attached to the driven shaft 21 b is not illustrated). Further, a structure is formed in which one end of the driven shaft 21 b is not connected to the motor, but is supported by the bearing. Accordingly, when the main shaft 21 a is rotated by the motor 23, the driven shaft 21 b is also rotated through the gear 24 and the another gear attached to the driven shaft 21 b.

Rotors 22 a to 22 c which are respectively accommodated inside the first to third pump chambers 16 a to 16 c and have a plurality of teeth are fixed and supported by the main shaft 21 a and the driven shaft 21 b (hereinafter, both will be referred to as the rotary shaft 21 if both shafts are not particularly distinguished from each other). The respective rotors 22 a to 22 c rotate along with the rotary shaft 21, and the gas which is suctioned from the intake holes 31 or the upstream communication holes 32 a or 32 b of the first to third pump chambers 16 a to 16 c is transferred to the downstream communication holes 32 a or 32 b or the exhaust holes 33 during the rotation of the rotors. Here, the rotors 22 a to 22 c are arranged with a small gap therebetween and the rotors 22 a to 22 c and the casing inner wall surface are arranged with a small gap therebetween.

Furthermore, the main shaft 21 a, the gear 24, the driven shaft 21 b, and the gear attached to the driven shaft 21 b constitute the support member. The support member is supported by the casing through the bearings 25 and 26. Further, the pairs of rotors 22 a to 22 c which are supported by the main shaft 21 a and the driven shaft 21 b inside the respective pump chambers 16 a to 16 c constitute the gas transfer member.

The exhaust hole 33 of the low pressure side roots pump 10L is connected to the intake hole 31 of the high pressure side roots pump 10H by the connection passage 41. It is desirable that the connection passage 41 is connected to a truncated conical tube so that the cross-sectional area perpendicular to the length direction decreases from the low pressure side roots pump 10L toward the high pressure side roots pump 10H. When the truncated conical tube is used, the flow rate of the gas increases and the temperature of the gas decreases due to the Venturi effect at the portion of which the cross-sectional area perpendicular to the length direction of the connection passage 41 is small, that is, the diameter of the cross-section is small. Since the pressure and the temperature are proportional to each other, the temperature of the gas decreases with a decrease in pressure. Further, since the outer periphery of the connection passage 41 contacts the external air, an increase in temperature may be suppressed. Furthermore, it is desirable that the connection passage 41 connect the low pressure side roots pump 10L to the high pressure side roots pump 10H at the shortest distance. Here, the shortest distance indicates a case where the connection passage 41 is disposed between the casing of the low pressure side roots pump 10L and the casing of the high pressure side roots pump 10H so that the length direction of the connection passage 41 is perpendicular to the rotary shaft 21, that is, the length direction is not deviated from the perpendicular direction.

FIGS. 3A and 3B are diagrams schematically illustrating a state of the temperature of the pump chamber during the operation of the roots pump, where FIG. 3A is a diagram illustrating a state where the temperature increases when the roots pump according to the embodiment is formed as double stages and FIG. 3B is a diagram illustrating a state where the temperature increases when multiple stages of pump chambers are provided in one of the roots pumps of the related art. In these drawings, the horizontal axis indicates the distance from the exhaust subject (chamber), and it becomes closer to the rear-stage (downstream) pump chamber as it goes to the right side. Further, the vertical axis indicates the temperature. Further, in FIGS. 3A and 3B, the number of stages of the pump chambers is equal to each other.

As illustrated in FIG. 3A, when the roots pump 10 is formed as double stages with the low pressure side roots pump 10L and the high pressure side roots pump 10H as in the embodiment and both pumps are connected to each other by the connection passage 41, the temperature increases while the gas is transferred inside the low pressure side roots pump 10L. However, when the gas flows from the low pressure side roots pump 10L to the high pressure side roots pump 10H through the connection passage 41 formed as the truncated conical tube, the flow rate increase and the temperature of the gas decreases. Subsequently, the temperature increases again while the gas is transferred inside the high pressure side roots pump 10H.

Meanwhile, as illustrated in FIG. 3B, when the exhaust is performed by one roots pump in which the plurality of pump chambers are connected in series to each other by the communication hole, the temperature of the gas increases while the gas is transferred inside the pump chambers of the roots pump. Then, the temperature of the gas at the final-stage pump chamber is higher than the temperature of the final-stage pump chamber at the high pressure side roots pump 10H of the embodiment.

In this way, the temperature of the gas which is exhausted from the roots pumps when causing the gas to pass through the identical-stage pump chambers becomes lower in the roots pump 10 according to the embodiment compared to the roots pump of the related art. The reason is as below. The roots pump 10 according to the embodiment is formed as double stages with the low pressure side roots pump 10L and the high pressure side roots pump 10H, and the connection passage 41 which is formed as the truncated conical tube is provided therebetween. Accordingly, the temperature of the gas decreases at the connection passage 41 and the gas of which the temperature decreases is supplied again to the high pressure side roots pump 10H.

Since an increase in temperature at the downstream side is suppressed in the low pressure side roots pump 10L even by the number of stages of the pump chambers compared to the roots pump used in the related art, the corrosion caused by the corrosive gas is prevented by the suppressed degree. As a result, ductile cast iron or the like may be used in the casing, the rotary shaft 21, or the rotors 22 a to 22 c. Further, in the high pressure side roots pump 10H, ductile cast iron subjected to the Ni-plating may be used in the casing, the rotary shaft 21, or the rotors 22 a to 22 c so as to suppress the generation of the corrosion caused by the corrosive gas due to an increase in temperature at the downstream side. The thickness of the Ni-plating may be, for example, 15 to 30 μm.

Next, an operation of the roots pump with such a configuration will be described. When the main shafts 21 a of the low pressure side roots pump 10L and the high pressure side roots pump 10H are respectively rotated by driving the motors 23, the rotors 22 a to 22 c supported by the main shaft 21 a rotates, and the driven shaft 21 b also rotates by the gear 24 connected thereto, so that the rotors supported by the driven shaft 21 b also rotate. The rotation direction of the rotor which is supported by the driven shaft 21 b becomes opposite to the rotation direction of the rotor which is supported by the main shaft 21 a. By the rotation of the rotors 22 a to 22 c, the gas inside the first to third pump chambers 16 a to 16 c is transferred to the communication holes 32 a and 32 b or the exhaust hole 33 at the downstream side. Then, the transferred gas is compressed in response to the ratio between the volumes of the first to third pump chambers 16 a to 16 c.

Specifically, the gas inside the exhaust subject flows from the intake hole 31 of the low pressure side roots pump 10L into the first pump chamber 16 a, and the gas is transferred to the second pump chamber 16 b through the communication hole 32 a by the rotation of the rotor 22 a. Since the volume of the second pump chamber 16 b is smaller than the volume of the first pump chamber 16 a, the gas which is transferred to the second pump chamber 16 b is compressed so that the temperature thereof increases. Further, the gas is further transferred to the third pump chamber 16 c with the smaller volume by the rotation of the rotor 22 b so that the temperature thereof increases. Subsequently, the gas is exhausted from the third pump chamber 16 c to the truncated conical tube by the rotation of the rotor 22 c. At this time, since the truncated conical tube becomes narrower toward the high pressure side roots pump 10H, the flow rate of the gas increases and the temperature of the gas decreases due to the Venturi effect. Further, since the periphery of the truncated conical tube contacts the atmosphere, the heat is also emitted into the atmosphere through the truncated conical tube, so that the temperature of the gas decreases. In a state where the temperature of the gas decreases as described above, the gas flows from the intake hole 31 of the high pressure side roots pump 10H into the first pump chamber 16 a. Even in the high pressure side roots pump 10H, as in the case of the low pressure side roots pump 10L, the gas is transferred in a compressed state from the first pump chamber 16 a to the third pump chamber 16 c and is exhausted from the exhaust hole.

Furthermore, as for the capacities of the respective motors 23 of the low pressure side roots pump 10L and the high pressure side roots pump 10H, the capacity of the low pressure side roots pump 10L having a low pressure becomes small, and the capacity of the high pressure side roots pump 10H having a high pressure becomes large. Further, it is possible to decrease the capacities of the respective motors 23 of the low pressure side roots pump 10L and the high pressure side roots pump 10H compared to the capacity of the motor of the existing roots pump in which N number of pump chambers are connected in series to each other, and hence to decrease the capacity of the motor even in total compared to the capacity of the motor of the roots pump of the related art.

Further, in the description above, a case has been described in which both the low pressure side roots pump 10L and the high pressure side roots pump 10H are formed as triple stages of pump chambers, but the roots pump having a single stage or more of pump chambers may be used.

In the embodiment, as not in the roots pump (hereinafter, referred to as the existing roots pump) in which N number of stages (N is a natural number equal to or larger than 2) of the pump chambers are connected in series to each other, two roots pumps, that is, the low pressure side roots pump 10L and the high pressure side roots pump 10H each including the rotary shaft 21 are provided and both roots pumps are connected to each other through the connection passage 41 by forming N number of stages of the pump chambers so as to have the same number of pump chambers in total. As a result, the length of the rotary shaft 21 constituting each of the low pressure side roots pump 10L and the high pressure side roots pump 10H is shortened compared to the existing roots pump. As described above, since an increase in the temperature of each of the roots pumps 10L and 10H is also small compared to the existing roots pump, the deformation of the rotary shaft 21 is suppressed, and hence there is an effect that the contact between the rotors 22 a to 22 c and the casing may be prevented.

Further, in a case where the roots pump 10 is used for the exhaust of the corrosive gas, the deterioration occurs in the high pressure side roots pump 10H in which the temperature of the gas increases. At this time, since only the rotors 22 a to 22 c of the high pressure side roots pump 10H in which the corrosion actively occurs may be replaced, there is also an effect that the maintenance cost may be decreased compared to the existing roots pump in which all rotors need to be replaced.

In addition, since the exhaust hole 33 of the low pressure side roots pump 10L is connected to the intake hole 31 of the high pressure side roots pump 10H by the truncated conical tube of which the cross-sectional area decreases as it goes toward the high pressure side roots pump 10H, it is possible to decrease the temperature of the gas by the Venturi effect and the external cooling when the gas passes through the truncated conical tube. Accordingly, since the temperature of the gas at the final-stage pump chamber 16 c of the high pressure side roots pump 10H may be lower than the temperature of the gas at the final-stage pump chamber of the existing roots pump, there is an effect that the corrosion of the rotor inside the first to third pump chambers 16 a to 16 c which increase in temperature and pressure may be suppressed compared to the related art.

Furthermore, in the embodiment, an example has been described in which two roots pumps, that is, the low pressure side roots pump 10L and the high pressure side roots pump 10H are used, but three or more roots pumps which are respectively connected to each other by the connection passage 41 may be used.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

What is claimed is:
 1. A roots pump comprising: a first roots pump including a first casing which has therein a first pump chamber, a first gas transfer member which is disposed inside the first pump chamber and rotates so as to transfer a gas in a predetermined direction, and a first support member which supports the first gas transfer member; a second roots pump including a second casing which has therein a second pump chamber, a second gas transfer member which is disposed inside the second pump chamber and rotates so as to transfer a gas in a predetermined direction, and a second support member which supports the second gas transfer member; and a connection passage connecting an exhaust hole which is provided in the first pump chamber to an intake hole which is provided in the second pump chamber.
 2. The roots pump according to claim 1, wherein in the connection passage, a cross-sectional area perpendicular to a gas transfer direction decreases as it goes from the first roots pump toward the second roots pump.
 3. The roots pump according to claim 1, wherein the first casing, the first gas transfer member, and the first support member are formed of a material without corrosion resistance, and the second casing, the second gas transfer member, and the second support member are formed of a material with corrosion resistance.
 4. The roots pump according to claim 1, wherein the first pump chamber has a plurality of pump chambers, the second pump chamber has a plurality of pump chambers, and the connection passage is connected between a pump chamber constituting the first pump chamber at the most downstream side in a gas transfer direction of the first roots pump and a pump chamber constituting the second pump chamber at the most upstream side in a gas transfer direction of the second roots pump.
 5. The roots pump according to claim 4, wherein the pump chambers constituting the first pump chamber of the first roots pump are formed so that the volume becomes smaller as it goes toward the downstream side in the gas transfer direction, and the pump chambers constituting the second pump chamber of the second roots pump is formed so that the volume becomes smaller as it goes toward the downstream side in the gas transfer direction and the volume of the pump chamber constituting the second chamber at the most upstream side in the gas transfer direction of the second roots pump becomes smaller than that of the pump chamber constituting the first chamber at the most downstream side in the gas transfer direction of the first roots pump.
 6. The roots pump according to claim 1, wherein the first roots pump further includes a first motor which rotates the first support member, the second roots pump further includes a second motor which rotates the second support member, and the capacity of the first motor is smaller than the capacity of the second motor.
 7. The roots pump according to claim 1, wherein the first support member includes a pair of rotary shafts which is a first rotary shaft and a second rotary shaft rotated, the first support member being supported by the first casing, the first gas transfer member includes a pair of rotors which is a first rotor and a second rotor respectively supported by the first rotary shaft and the second rotary shaft, the second support member includes a pair of rotary shafts which is a third rotary shaft and a fourth rotary shaft rotated, the second support member being supported by the second casing, and the second gas transfer member includes a pair of rotors which is a third rotor and a fourth rotor respectively supported by the third rotary shaft and the fourth rotary shaft.
 8. The roots pump according to claim 7, wherein a first motor which rotates the first rotary shaft is connected to the first rotary shaft, the first rotary shaft and the second rotary shaft are connected to each other through gears so as to rotate in the opposite directions, a second motor which rotates the third rotary shaft is connected to the third rotary shaft, and the third rotary shaft and the fourth rotary shaft are connected to each other through gears so as to rotate in the opposite directions.
 9. The roots pump according to claim 8, wherein the capacity of the first motor is smaller than the capacity of the second motor.
 10. The roots pump according to claim 1, wherein the connection passage is disposed so that a length direction of the connection passage is perpendicular to a length direction of the first support member and the second support member.
 11. The roots pump according to claim 3, wherein the first casing, the first support member, and the first gas transfer member are formed of ductile cast iron, and the second casing, the second support member, and the second gas transfer member are formed of ductile cast iron subjected to Ni-plating.
 12. The roots pump according to claim 7, wherein a small gap is formed between the first rotor and the second rotor, between the first rotor and an inner wall surface of the first casing, between the second rotor and the inner wall surface of the first casing, between the third rotor and the fourth rotor, between the third rotor and an inner wall surface of the second casing, and between the fourth rotor and the inner wall surface of the second casing.
 13. The roots pump according to claim 2, wherein the connection passage is formed as a truncated conical tube.
 14. The roots pump according to claim 1, further comprising: N−2 number of k-th roots pumps including a k-th casing which has therein a k-th pump chamber, a k-th gas transfer member which is disposed inside the k-th pump chamber and rotates so as to transfer a gas in a predetermined direction, and a k-th support member which supports the k-th gas transfer member when N is set to an integer equal to or larger than 3 and k is set to an integer from 3 to N, wherein the connection passage is provided between the respective roots pumps so as to connect an exhaust hole which is provided in the k−1-th pump chamber to an intake hole which is provided in the k-th pump chamber.
 15. The roots pump according to claim 7, wherein the first rotor, the second rotor, the third rotor, and the fourth rotor includes a plurality of teeth.
 16. An exhaust method comprising: transferring a gas, which is suctioned from a first intake hole of a first casing inside the first casing having a first pump chamber, to a first exhaust hole of the first casing while compressing the gas inside the first pump chamber; transferring the gas which is exhausted from the first exhaust hole to a second intake hole of a second casing having a second pump chamber; transferring the gas which is suctioned from the second intake hole inside the second casing to a second exhaust hole of the second casing while compressing the gas inside the pump chamber; and discharging the gas from the second exhaust hole.
 17. The exhaust method according to claim 16, wherein the transferring gas from the first exhaust hole to the second intake hole is performed so as to increase the flow rate of the gas and to decrease the temperature of the gas.
 18. The exhaust method according to claim 16, wherein the transferring the gas inside the first casing having the first pump chamber is performed by a rotation of a pair of rotors which has a plurality of teeth and is supported by a pair of rotary shafts rotated is supported by the first casing, and the transferring the gas inside the second casing having the second pump chamber is performed by a rotation of a pair of rotors which has a plurality of teeth and is supported by a pair of rotary shafts rotated is supported by the second casing.
 19. The exhaust method according to claim 15, wherein the first pump chamber has a plurality of pump chambers, the second pump chamber has a plurality of pump chambers, and in the transferring the gas from the first exhaust hole to the second intake hole, the gas is transferred from a pump chamber constituting the first pump chamber at the most downstream side in a gas transfer direction of the first roots pump to a pump chamber constituting the second pump chamber at the most upstream side in a gas transfer direction of the second roots pump.
 20. The exhaust method according to claim 19, wherein the pump chambers constituting the first pump chamber of the first roots pump are formed so that the volume becomes smaller as it goes toward the downstream side in the gas transfer direction, and the pump chambers constituting the second pump chamber of the second roots pump are formed so that the volume becomes smaller as it goes toward the downstream side in the gas transfer direction and the volume of the pump chamber constituting the second pump chamber at the most upstream side in the gas transfer direction of the second roots pump becomes smaller than that of the pump chamber constituting the second pump chamber at the most downstream side in the gas transfer direction of the first roots pump. 